1. Field of the Invention
The present invention relates to a shock absorber adapted to be mounted between a body and a wheel shaft in a vehicle such as a car or a motorcycle for reducing shocks and vibrations derived from a road.
2. Description of the Prior Art
In many of the conventional shock absorbers, a cushion means is used in order to prevent a piston from violently striking a bearing or a cylinder head when the piston is fully expanded.
For example, it is known to mount a rebound cushion such as one formed of rubber positioned on a periphery of a piston rod; as one formed the piston approaches the end of the expanding stroke, the rebound cushion is brought into contact with the bearing to minimize the impact which the bearing might receive from the piston. A common problem with these conventional shock absorbers is that since the rebound cushion is made of a relatively hard material to last long, an unpleasant impact sound is still created when the rebound cushion strikes the bearing.
Meanwhile, an oil-lock shock absorber has been proposed in my coassigned Japanese Patent Laid-open Publication No. 196432/1987. As illustrated in FIG. 3 of the accompanying drawing, this shock absorber comprises a cylinder 10, a piston rod 12 having a piston 11 movably received in the cylinder 10 so as to divide the cylinder bore into upper and lower oil chambers A, B, a tubular sub-cylinder 18 of circular cross section mounted in the cylinder 10 at one end thereof, and an oil-lock valve 13 mounted on a periphery of the piston rod 12.
The valve 13 of the prior art shock absorber includes a stop having a through-hole 23 and retained by a snap ring 21, a floating casing 20 covering the stop 19, and a leaf valve member 22 disposed at the lower side of the through-hole 23. When the piston approaches the end of expanding stroke, the valve 13 is brought into the sub-cylinder 18 to compress the oil in the sub-cylinder 18. Then the compressed oil flows into the lower oil chamber A via the through-hole 23 and the leaf valve 22. Due to the action of the leaf valve member 22 and the flow resistance of the through-hole 23, an oil-lock load can be achieved.
However, also in this prior oil-lock shock absorber, on unpleasant impact sound is created as the valve 13 strikes the bearing. Further, the adjustment of the oil-lock load can be accomplished only by varying the size of the through-hole 23. Consequently the oil-lock load value is determined due to the characteristic of square of the orifice and hence varies widely, which would cause a staggered oil-lock load.